The recent widespread use of wireless microphone systems in the entertainment field, has revealed several problematic areas that remain to be addressed and rectified. For instance, entertainers travel extensively, and perform on a multitude of stages, in diverse environmental surroundings. Each stage or other setting for the performance can present unique electrical interference conditions, which can severely and adversely affect the reception of the voice signals being transmitted over the short range wireless microphone radio system. In this regard, there are many spurious background signals present in any environment, especially where other electrical equipment is being used, and where there are other broadcast signals present.
One of the attempted solutions for overcoming the background interference problems, has been the provision of a pair of carrier frequencies, to enable the user to have a choice of carrier frequencies, in an attempt to find the one which is less affected by spurious interference signals.
As a further attempt to overcome the background interference problem, a group or spectrum of carrier frequencies have been employed. The additional frequencies provide a larger selection of frequencies from which to choose. However, this attempted solution has proven to be less than entirely satisfactory. Side-band producing multiplication stages are used in the modulation process, prior to transmission over an antenna, to achieve the larger number of carrier frequencies, and thus additional interference is introduced by such multiplication stages.
The conventional multiplication stages have been used, since conventional oscillators oftentimes include quartz crystals, which oscillate at relatively low frequencies. For instance, a quartz crystal typically oscillates at about 20 Megahertz. This frequency is multiplied by a desired factor (e.g., 8, 9, or 12), in order to achieve predetermined, authorized VHF or UHF carrier frequencies. However, each of the several multiplication stages generates undesired and unwanted side bands and harmonic signals, which adversely affect the quality of the transmitted signals, and interfere with other short range radio communication systems. Furthermore, each multiplication stage increases the physical size and expense of the communication systems. Such a system is proportionately more susceptible to malfunction than the dual frequency units, due to the increased number of electronic components used in the multiple frequency units.
Such side band problem is compounded where two or more wireless microphone systems operate simultaneously with the same receiver system, at different carrier frequencies. In such a multiple system, the undesired side bands and harmonic frequencies produced by one wireless microphone system can, and frequently does cause unwanted interference with other simultaneously operating microphone systems.
The problem is further complicated by crowded and limited authorized carrier frequencies, which compound frequency incompatibility. Numerous attempts have been made to overcome this problem. For instance, reference may be made to an article entitled "An Introduction to Wireless Microphone-Multiple System Frequency Compatibility ", by Kenneth Fasen, presented at the 81st convention of the Audio Engineering Society, on Nov. 12, 1986 at Los Angeles, Calif. The article describes the causes of such frequency incompatibility, and suggests remedial techniques therefor. While these techniques have been successful for many applications, it would be highly desirable to avoid any and all of the multiple system incompatibility problems entirely, without the need to employ any remedial techniques.
Therefore, it would be highly desirable to have a short range radio communication system which greatly reduces, or even eliminates entirely, the side band and harmonic frequency interference problems, caused by successive multiplication stages. It would also be desirable to modulate directly the audio frequencies to predetermined VHF and UHF carrier frequencies, without the need for multiplier stages. Additionally, it would be highly desirable to reduce the size of the radio communication system, while rendering it lighter in weight to carry, more rugged to handle, and less expensive to manufacture. At the same time, such a communication system would be capable of functioning in a multiple system, without the need for remedial techniques for eliminating the foregoing mentioned interference problems associated with such multiple systems.